Pim kinases are family of three highly-related serine and threonine protein kinases encoded by the genes Pim-1, Pim-2, and Pim-3. The gene names are derived from the phrase Proviral Insertion, Moloney, frequent integration sites for murine moloney virus wherein the insertions lead to overexpression of Pim kinases and either de novo T-cell lymphomas, or dramatic acceleration of tumorigenesis in a transgenic Myc-driven lymphoma model (Cuypers et al. (1984) Cell, vol. 37 (1) pp. 141-50; Selten et al. (1985) EMBO J. vol. 4 (7) pp. 1793-8; van der Lugt et al. (1995) EMBO J. vol. 14 (11) pp. 2536-44; Mikkers et al. (2002) Nature Genetics, vol. 32 (1) pp. 153-9; van Lohuizen et al. (1991) Cell, vol. 65 (5) pp. 737-52). These experiments reveal synergy with the oncogene c-Myc, and suggest that inhibition of the Pim kinases may have therapeutic benefit.
Mouse genetics suggests that antagonizing Pim kinases may have an acceptable safety profile; a Pim 1−/−; Pim-2−/−, Pim-3−/− mouse knockout is viable although slightly smaller than wild type littermates (Mikkers et al. (2004) Mol Cell Biol vol. 24 (13) pp. 6104-154). The three genes give rise to six protein isoforms including a protein kinase domain, and apparently without recognizable regulatory domains. All six isoforms are constitutively active protein kinases that do not require post-translational modification for activity, thus Pim kinases are regulated primarily at the transcriptional level (Qian et al. (2005) J Biol Chem, vol. 280 (7) pp. 6130-7). Pim kinase expression is highly inducible by cytokines and growth factors receptors and Pims are direct transcriptional targets of the Stat proteins, including Stat3 and Stat5. Pim-1, for example, is required for the gp130-mediated Stat3 proliferation signal (Aksoy et al. (2007) Stem Cells, vol. 25 (12) pp. 2996-3004; Hirano et al. (2000) Oncogene vol. 19 (21) pp. 2548-56; Shirogane et al. (1999) Immunity vol. 11 (6) pp. 709-19).
Pim kinases function in cellular proliferation and survival pathways parallel to the PI3k/Akt/mTOR signaling axis (Hammerman et al. (2005) Blood vol. 105 (11) pp. 4477-83). Indeed, several of the phosphorylation targets of the PI3k axis including Bad and eIF4E-BP1 are cell growth and apoptosis regulators and are also phosphorylation targets of the Pim kinases (Fox et al. (2003) Genes Dev vol. 17 (15) pp. 1841-54; Macdonald et al. (2006) Cell Biol vol. 7 pp. 1; Aho et al. (2004) FEBS Letters vol. 571 (1-3) pp. 43-9; Tamburini et al. (2009) Blood vol. 114 (8) pp. 1618-27). Pim kinase may affect cell survival since phosphorylation of Bad increases Bcl-2 activity and therefore promotes cell survival. Likewise, phosphorylation of eIF4E-BP1 by mTOR or Pim kinases causes depression of eIF4E, promoting mRNA translation and cellular growth. In addition, Pim-1 has been recognized to promote cell cycle progression through phosphorylation of CDC25A, p21, and Cdc25C (Mochizuki et al. (1999) J Biol Chemvol. 274 (26) pp. 18659-66; Bachmann et al. (2006) Int J Biochem Cell Biol vol. 38 (3) pp. 430-43; Wang et al. (2002) Biochim Biophys Acta vol. 1593 (1) pp. 45-55.
Pim kinases show synergy in transgenic mouse models with c-Myc-driven and Akt-driven tumors (Verbeek et al. (1991) Mol Cell Biol vol. 11 (2) pp. 1176-9; Allen et al. Oncogene (1997) vol. 15 (10) pp. 1133-41; Hammerman et al. (2005) Blood vol. 105 (11) pp. 4477-83). Pim Kinases are involved in transforming activity of oncogenes identified in acute myeloid leukemia (AML) including Flt3-ITD, BCR-abl, and Tel-Jak2. Expression of these oncogenes in BaF3 cells results in upregulation of Pim-1 and Pim-2 expression, resulting in IL-3 independent growth, and subsequent Pim inhibition results in apoptosis and cell growth arrest (Adam et al. (2006) Cancer Research vol. 66 (7) pp. 3828-35). Pim overexpression and dysregulation has also been noted as a frequent event in many hematopoietic cancers, including leukemias and lymphoma (Amson et al. (1989) Proc Natl Acad Sci USA vol. 86 (22) pp. 8857-61); Cohen et al. (2004) Leuk Lymphoma vol. 45 (5) pp. 951-5; Hüttmann et al. (2006) Leukemia vol. 20 (10) pp. 1774-82) as well as multiple myeloma (Claudio et al. (2002) Blood vol. 100 (6) pp. 2175-86. Pim 1 has been shown to be overexpressed and correlated to prostate cancer progression (Cibull et al. (2006) J Clin Pathol vol. 59 (3) pp. 285-8; Dhanasekaran et al. (2001) Nature vol. 412 (6849) pp. 822-6). Pim 1 expression increases in mouse models with disease progression (Kim et al. (2002) Proc Natl Acad Sci USA vol. 99 (5) pp. 2884-9). Pim-1 has been reported to be the most highly overexpressed mRNA in the subset of human prostate tumor samples which have a c-Myc-driven gene signature (Ellwood-Yen et al. (2003) Cancer Cell vol. 4 (3) pp. 223-38). Pim-3 has been also been shown to be overexpressed and to have a functional role in pancreatic cancer and hepatocellular carcinoma (Li et al. (2006) Cancer Research vol. 66 (13) pp. 6741-7; Fujii et al. (2005) Int J Cancer, vol. 114 (2) pp. 209-18.
Beyond oncology therapeutic and diagnostic applications, Pim kinases could play an important role in normal immune system function and Pim inhibition could be therapeutic for a number of different immunologic pathologies including inflammation, autoimmune conditions, allergy, and immune suppression for organ transplantation (Aho et al. Expression of human Pim family genes is selectively up-regulated by cytokines promoting T helper type 1, but not T helper type 2, cell differentiation. Immunology (2005) vol. 116 (1) pp. 82-8).